JPS6348025B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a laser ranging device suitable for long-distance ranging such as geodetic measurement and observation of crustal deformation.

[Prior art]

BACKGROUND ART Conventionally, pulsed laser distance measuring devices have been widely used for long distance measuring.

As is known from the examples of artificial satellite systems and lunar ranging systems, this method uses a time interval counter to measure the time it takes for a high-output pulsed laser beam to travel back and forth to the target, and then calculates the distance to the target. However, its accuracy is limited to a few centimeters.

Incidentally, in recent years, in geodetic systems for the purpose of earthquake prediction, etc., it has become necessary to be able to detect distance displacements of about 1 cm per year over an interval of several tens of kilometers.

On the other hand, as mentioned above, conventional pulsed laser type laser distance measuring devices are not capable of sufficiently meeting purposes such as earthquake fluctuation observation in terms of the required measurement accuracy.

The reason for this is that according to current laser technology, the pulse width ranges from several ps to several tens of ps (converted to a length of several hundred μm to several tens of ps).
Despite the fact that a powerful pulsed laser beam of mm) is obtained, the response of the photomultiplier tube, pulse amplifier, threshold circuit, and time interval counter system, which are the main components of a laser rangefinder, is This is because the speed is not high enough to faithfully process laser light, and there are uncertain factors such as delays and time jitter in these circuit systems. That is, at present, the best pulse width of pulsed laser light is several hundred ps (several centimeters in terms of length).

Note that some laser distance measuring devices use a laser modulation method, and are known by names such as geodimeters and light wave distance meters. Although its accuracy is quite high, it can only be applied to relatively short distance measurements (less than several tens of kilometers), and cannot be used for crustal deformation observation as described above.

[Purpose of the invention]

An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a laser ranging device that can directly measure distances over long distances, such as crustal deformation observation, with desired high accuracy.

[Summary of the invention]

The configuration of the laser distance measuring device according to the present invention includes a pulsed laser oscillator that emits a pulsed laser beam of a predetermined width, and a back-and-forth reflection that uses laser beam interference to control and maintain the interval at a predetermined value with a predetermined accuracy. a streaky camera system that detects the time difference between the incident pulsed laser beams, and a streaky camera system that receives the pulsed laser beams emitted from the pulsed laser oscillator and transmits the pulsed laser beams to the target to be ranged and the above-mentioned cavity resonant system. In addition to branching and projecting against
It includes at least a transmitting/receiving optical system that allows each reflected light of the pulsed laser light from them to enter the streak tube camera system, and the pulsed laser light reflected from the target reciprocates within the cavity resonant system. The reflected pulsed laser beam and the extracted pulsed laser beam are detected and identified, and the distance to the target is determined by the distance corresponding to the time difference between the two pulsed laser beams and the round trip distance of the pulsed laser beam within the cavity resonator. This allows distances to be calculated and measured.

In short, by using the above-mentioned cavity resonance system as a standard for absolute distance related to distance measurement and combining it with a streak camera system, distortion of the measuring device can be reduced.
This minimizes jitter, physically determines the amount of delay, and makes full use of the characteristics of extremely short pulsed laser light, making it possible to achieve high precision in long-distance distance measurement.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a structural diagram of an embodiment of a laser ranging device according to the present invention, and FIG. 2 is a waveform diagram of a cavity resonant output optical pulse train.

Here, 101 is a pulse laser oscillator, 102
102-1, 102-2 are pulsed laser beams output (emitted) from the pulsed laser oscillator 101, 102-1, 102-2 are branched pulsed laser beams, 102-3, 102-4
103 is a reflected pulsed laser beam, and 103 is an optical beam splitter related to the transmitting/receiving optical system.

104 is a cavity resonator type mirror device related to a cavity resonance system, which is composed of the following parts 104-1 to 104-5,
104-1, 104-2 are flat reflecting mirrors, 104-
3 is a mirror support having a displacement correction drive mechanism;
104-4 is a fixed mirror support, and 104-5 is a vacuum chamber for keeping the optical path length constant. 1
05 is a laser interferometer also related to a cavity resonance system, 105-1 to 105-5 are its optical systems, 105-1 is a corner cube mirror, 105-2
is a right angle prism, 105-3 is a beam splitter, 105-4 is a plane mirror, and 105-5 is a lens. Also, 106 is a laser oscillator for wavelength stabilization of the laser interferometer 105, which also relates to the cavity resonance system, 107 is a photodetector, 108 is a phase detector, and 109 is an amplifier. Note that 110 is a servo driver for the cavity resonant mirror device 104.

111 is a condensing lens related to the transmission/reception optical system; 112
is the same beam splitter, 113 is the same lens, 1
14 is the same photodetector, 115 is the same gain control amplifier,
116 is the same gate circuit, 117 is the same pulse counter, 118 and 119 are the same reflecting mirrors, 120 is the same lens, 121 is the same detector, 122 is the same electric shutter, 123 is the same slit, and 124 is the same lens.

125 is a streak tube related to the streak tube camera system; 125-1 is a photocathode thereof; 125-2 is a deflection plate thereof; 125-3 is a channel type photomultiplier plate; 125-4 is a fluorescent plate; 127 is the same sweep voltage generation circuit, 127 is the same lens, 128 is the same type of imaging camera, 129
is also an analyzer, 130 is also a display,
131 and 132 indicate the signal images.

Note that 133 is a target reflector to be measured.

First, the pulsed laser beam 102 output from the pulsed laser oscillator 101 is transmitted to the beam splitter 1
03, it is branched into 102-1 and 102-2. The pulsed laser beam 102-1 is directed to the target 133.
The light 102-2 is branched so as to be perpendicularly incident on the plane of the plane reflecting mirror 104-1 of the cavity resonator mirror device 104.

The pulsed laser beam 102-3 reflected by the target reflector 133 enters the condenser lens 111. On the other hand, the light entering the plane reflecting mirror 104-1 is repeatedly reflected back and forth between the plane reflecting mirror 104-2, which faces the plane reflecting mirror 104-2 at a distance L. Plane reflector 104-1
is designed to allow a part of the light to pass through, and each time the pulsed laser light goes back and forth between the plane reflecting mirrors 104-1 and 104-2, a part of it is emitted to the outside as reflected pulsed laser light 102-4. . As a result, a light pulse train as shown in FIG.
-1 is output.

Distance L between plane reflecting mirrors 104-1 and 104-2
is a laser interferometer 105 and its attached circuit 106
~109 and the servo driver 110, the length is precisely controlled and maintained at a constant length. The interferometer accuracy is
It mainly depends on the wavelength stability of the laser oscillator 106, which is currently about 10 ^-8 to 10 ^-9 at the product level. In this way, the incident pulsed laser beam 104-2
is subjected to laser beam interference in the cavity resonant mirror device 104 as described above, and then output as a reflected pulsed laser beam 102-4.

Output light 102-4 from plane reflecting mirror 104-1
is reflected by the beam splitter 103 and enters the lens 111 in the same way as the reflected light from the target reflector 133.

A part of the light incident on the condensing lens 111 is split by the beam splitter 112, and then the light enters the lens 113.
The light is condensed and incident on the detector 114.

As shown in FIG. 2, the peak value of the output optical pulse train from the plane reflecting mirror 104-1 gradually decreases due to diffraction, reflection, and exit loss during each round trip. The state of this loss and reduction is determined by the parameters of the device, that is, the plane reflecting mirrors 104-1 and 104-2.
It can be calculated theoretically if the reflectance of , the mirror interval (reciprocating interval) L, the diameter and spread angle of the incident pulsed laser beam 102-2, etc. are given. Ultimately, it is also possible to confirm experimentally.

The power of the incident pulsed laser beam 102-2 is P ₀ ,
If the reflectance of the flat reflecting mirror 104-1 to the incident pulsed laser beam is R ₁ (transmittance is 1-R ₁ ), and the reflectance of the flat reflecting mirror 104-2 is R ₂ , then after n times of reflection, The output P _{o of} is given by the following equation.

P _o = P ₀ (1-R ₁ ) ² R ^n-1 ₁ R ⁿ ₂ ......(1) If reflectance R ₂ is 100%, P _o = P ₀ (1-R ₁ ) ² R ^{n -1} ₁ ………(2) Also, the plane reflector 1 whose output is the maximum at the nth time
It can be seen that the condition for the reflectance of 04-1 is R ₁ =(n-1)/(n+1) (3) from ∂P _o /∂R ₁ =0.

Furthermore, regarding diffraction loss, for example, if the plane reflecting mirror 104-1 is circular, its diameter is D ₁ , the diameter of the incident laser beam is d, and its divergence angle is θ _t , then after n times of reflection, the plane reflecting mirror 104 - The rate α of the amount of light within a magnitude range of 1 is given as follows.

α=D ² /(d+2nLθ _t ) ² (4) However, assume that d+2nLθ _t >D ² .

In the case of a laser beam, the divergence angle θ _t can be set close to the divergence angle of only diffraction, so it is given by θ _t =2.44λ/d (5). Here, λ represents the wavelength of the laser beam.

As described above, according to formulas (1) to (5), etc., the plane reflecting mirror 104
It is possible to predict the change in the output peak value from -1.

The gain control amplifier 115 is an amplifier whose gain changes in response to the change in the peak value as described above, and is used to keep the peak value of the output electrical pulse constant.

That is, the pulsed laser light converted into an electric signal by the detector 114 enters the pulse counter 117 via the gain control amplifier 115 and the gate circuit 116. The first pulse is directly reflected light from the plane reflecting mirror 104-1, and the logic is such that this starts a counter and counts from the next pulse. The counterstop includes reflectors 118 and 119, a lens 120, and a detector 121.
This is done by the reflected pulsed laser light signal from the target reflector 133, which is detected by. That is, this count number becomes the number of round trip reflections n.

The output signal from the detector 121 is also used as a trigger signal for the sweep voltage generation circuit 126 of the streak tube 125. The generation of the sweep voltage is performed in synchronization with the reflected pulsed laser light from the target 133 reaching the photocathode 125-1 of the streak tube 125. Optimization of this synchronization is possible by changing the delay time by changing the interval between the reflecting mirrors 118 and 119 or the cable length. The length of the sweep is at least as long as the target reflector 13
Planar reflector 10 with reflected pulsed laser light from 3
It is necessary that one output light from 4-1 be included in one sweep. To obtain even higher resolution, it is necessary that the two pulses be close together. This is possible by changing the position of target reflector 133.

The reflected light from the target reflector 133 and the plane reflecting mirror 104-1 passes through the slit 123 and the lens 124, and then reaches the photocathode 125- of the streak tube 125.
1. The optical pulses converted into an electron beam stream within the streak tube 125 are voltage swept and passed through the channel type photomultiplier plate 125-3.
is incident on the fluorescent surface 125-4 and is multiplied here.
is visualized in the image. It is known that the position of this image corresponds to the sweep time and that high temporal resolution can be obtained.

This image is detected by an imaging camera 128 through a lens 127, processed by an analyzer 129, and displayed on a display 130. This embodiment shows an example in which a streak image of the reflected light from the target reflector 133 and the output light from the plane reflecting mirror 104-1 is displayed as is.

The information required here is the arrival time difference between the two pulses. Now, let us assume that the pulse of the image shown in the signal image 131 is the n-th output pulse from the plane reflecting mirror 104-1, and the pulse of the image shown in the signal image 132 is the reflected pulse from the target reflector 133, and the time difference between them, that is, the corresponding If the distance difference is δ (displayed on display 130), then the distance ρ to the target can be expressed by the following equation, excluding the inherent delay in the system.

ρ=nL+δ/2 (6) By controlling the cavity resonant reciprocating interval L with the laser interferometer 105, it can be practically within an error of about 10 ^-8 , that is, an error of 0.5 mm for 50 km. It can be made constant like this. The resolution of the Streak Tube 125 is approximately 10 ps (3 mm in length) as a general product. On the other hand, the pulsed laser oscillator 101 is in a situation where it can output sufficiently high-power pulsed laser light with a pulse width of about 10 ps.

Therefore, with these features, it is possible to measure a desired long distance with a desired accuracy using the device configuration of the above embodiment.

In addition, in the above embodiment, the shutter 122
is placed in front of the streak tube 125 in order to protect the photocathode 121-1 of the streak tube 125 when the direct reflected light and the initial output light from the plane reflector 104-1 are strong.
This is to temporarily block them.

Finally, the feasibility of this device will be explained in more detail.

In the above embodiment, the pulse laser oscillator 10
1. The second harmonic light (wavelength) of the Nd-YAG laser
0.53μm), and this output is
10mJ/pulse. This is a laser beam with a power of 1 GW when the pulse width is 10 ps, and such a laser beam is fully achievable with current laser technology. The number of output photons (photons) per pulse of this laser light is approximately 2.7×10 ¹⁶ .

In the above embodiment, the plane reflecting mirror 104-1,
Assuming that the diameter D of 104-2 is 20cm, the interval L is 10m, and the diameter d of the incident laser beam is 5cm, 2nL = 100Km
(assuming distance to target ρ = 50km), plane reflector 1
Reflectance R ₁ of 04-1 = 99.9%, flat reflector 104
-2 reflectance R ₂ = 100%, the formula
Using (1) to (5), when calculating each attenuation rate due to loss after n = 5000 reflections, the attenuation rate due to reflection P _o /
P ₀ is 6.7×10 ^-9 , and the attenuation factor α due to diffraction is 5.7×10 ^-3
become. Multiplying these two attenuation rates is 3.8×
It becomes 10 ^-11 . This means that even if 10% of the laser output is branched and incident on the plane reflecting mirror 104-1,
Since the number of photons is 2.7×10 ¹⁵ , the photons are passed between the plane reflecting mirrors 104-1 and 104-2 n times (for example,
This shows that a pulse containing about 10 ⁵ photons can be obtained even after 5000 round trips). In other words, this shows that it is fully possible to construct a system capable of measuring distances of about 50 km using the system of the embodiment.

〔Effect of the invention〕

As explained in detail below, according to the present invention, the distance measuring device is quite large-scale, but it is especially suitable for long-distance observation such as crustal deformation observation that requires continuous observation at the same location over a long period of time. Direct distance measurement can be performed with the desired high accuracy, and the effect is remarkable. Because it uses a pulsed laser, it is also suitable for other long-distance distance measurements, making it possible to perform high-precision direct distance measurement over tens of kilometers, which is difficult to do with conventional geometers and light wave rangefinders. It's also effective.

[Brief explanation of drawings]

FIG. 1 is a structural diagram of an embodiment of a laser distance measuring device according to the present invention, and FIG. 2 is a waveform diagram of a cavity resonance output optical pulse train thereof. 101... Pulse laser oscillator, 102... Pulse laser beam, 103... Optical beam splitter,
104...Cavity resonator mirror device, 105...Laser interferometer, 106...Laser oscillator for wavelength stabilization, 1
07...Photodetector, 108...Phase detector, 109...
Amplifier, 110... Servo driver, 111... Condensing lens, 112... Beam splitter, 113...
Lens, 114... Photodetector, 115... Gain control amplifier, 116... Gate circuit, 117... Pulse counter, 118, 119... Reflector, 120... Lens, 121... Detector, 122... Electric shutter,
123...Slit, 124...Lens, 105...Streak tube, 126...Sweep voltage generation circuit,
127... Lens, 128... Imaging camera, 129...
Analyzer, 130... Display device, 131, 132...
Signal image, 133...Target reflector.

Claims (1)

[Claims] 1. A pulsed laser oscillator that emits a pulsed laser beam with a predetermined width, a cavity resonant system that uses laser beam interference to control and maintain the round-trip reflection interval at a predetermined value with a predetermined precision, and an incident pulsed laser beam. A streak tube camera system that detects the time difference between It includes at least a transmitting/receiving optical system that allows each reflected light of the pulsed laser light to enter the streak tube camera system, and the pulsed laser light reflected from the target is reflected back and forth by the cavity resonant system. The distance to the target is calculated based on the distance corresponding to the time difference between the two pulsed laser beams and the round trip distance of the pulsed laser beam within the cavity resonator. A laser distance measuring device that enables measurements.